We propose to develop a thermoacoustic computed tomography (TCT) scanner for localizing optical-dye-labeled molecular probes in laboratory mice in vivo. This device offers two significant improvements over fluorescence intensity measurements: 1. Fully three-dimensional images will be acquired using a single axial scan of a novel, cylindrical detector army. 2. Exogenous molecular probes, to which a cyanine dye has been attached, will be detected using a dual-wavelength imaging approach. In Phase I we intend to quantify the spatial resolution and low contrast detection limitations of a TCT device that incorporates a 128-element, cylindrical transducer array. These studies will be carried out in tissue-mimicking phantoms, and the results compared to those we have acquired using a TCT device that incorporates a conventional, linear transducer array. We will employ a dual-wavelength, differential-measurement technique for isolating two non-tumor specific carbocyanine dyes (indocyanine dye and cypate), and a tumor-specific optical imaging agent (cytate) with peak absorption at 790 nm. These probes have high extinction coefficient (> 150,000 M-1 cm-1) and low fluorescence quantum yield (<20%), characteristics that favor thermoacoustic imaging. Our goals are to: 1. achieve < 0.5-mm high-contrast, spatial resolution in three dimensions, and 2. image a dye concentration of < 100 nM with a signal-to-noise ratio (SNR) of > 5 within a 1 mm3 volume (100 fmoles) embedded in a tissue-mimicking phantom. In Phase II we will construct a prototype TCT scanner and site the device at the Optical Radiology Laboratory, Washington University, St. Louis, MO. There, researchers will utilize novel monomolecular multimodal imaging agents to compare the imaging capabilities of this TCT scanner to a commercial mu-PET scanner and conventional fluorescence intensity imaging system in a series of phantom and in vivo mouse-imaging experiments.